1. Field of the Invention
The present invention relates to phased array radars and, more particularly, to subarrays of the antennas of such radars.
2. Description of the Prior Art
In the prior art, phased array radars have been employed to provide a beam cluster antenna pattern which is centered on a known average direction, and also to provide a narrow beam antenna pattern suitable for scanning a limited range. In the beam cluster usage, the average direction of the beam cluster has been determined by phase shifters associated with each of the radiating elements in the antenna while the cluster pattern has been determined by a second phase shift mechanism. Typically, this second phase shift mechanism consists of transmission lines of varying lengths, associated with antenna subarrays comprised of a number of the antenna radiating elements. In the narrow beam usage, the direction of the single beam is also determined by phase shifters which, in this case, are associated directly with antenna subarrays comprised of a plurality of antenna radiating elements such that the phase shifters are controlled to regulate the scanning of the single beam.
Both of these prior art phased array radars have employed antenna subarrays comprised of agglomerated radiating elements which define geometrically regular patterns on the conducting surface of the radar antenna as a means to avoid the cost and unreliability associated with the apparatus necessary to individually control the phase and amplitude of each of the radiating elements. These geometrically regular subarrays provide an illumination phase and amplitude which precisely matches the phase and amplitude necessary to produce the illumination function required for the desired antenna pattern only at a single point on the subarray. The difference between the actual illumination function and the ideal illumination function needed to produce the desired pattern is known as the illumination error function; and the difference between the actual antenna pattern and the desired antenna pattern is given by the Fourier transform of the illumination error function.
For most prior art antenna subarrays, the resulting illumination error function changes sign at regular intervals, giving a periodicity in the error function which produces energy concentrations in the antenna pattern at specific angles from the direction normal to the conducting surface. Such energy concentrations are commonly referred to as sidelobes so that the antenna pattern, as modified by the illumination error is said to exhibit sidelobes at specific angles. To those skilled in the art, sidelobes produced by such periodic disturbances are also called grating lobes. Such grating lobes are undesirable in that they degrade the quality of the antenna pattern.
Prior art phased array radars which have provided an antenna pattern having low sidelobes have generally not employed the use of subarrays and incurred the consequent higher cost and lower reliability of phased array radars which control the excitation of the radiating elements individually.
Prior art phased array radars which employed the use of antenna subarrays have generally used geometrically regular subarrays which have a periodic illumination error function that permits large grating lobes to arise at specific angles in the antenna pattern. For example, since the desired amplitude distribution usually remains constant as the beam is steered, amplitude errors have been compensated by distributing the energy non-uniformly among the antenna elements in each subarray. However, because illumination phase errors are usually much larger than illumination amplitude errors the improvement in illumination function afforded by this embodiment has not been heretofore economically justifiable when the phase errors are otherwise uncompensated for. For radars whose pattern quality is to be measured by the peak value of its grating lobes, these prior art antennas employing the use of subarrays did not provide acceptable performance for many applications.
Other prior art phased array radars employing antenna subarrays have limited the magnitude of the antenna grating lobes through the selection of the location of the antenna elements on the face of the antenna. These antennas, however, have resulted in relatively complex arrangements of antenna elements in comparison to the regular grid arrangement, and even more complex computations as to the appropriate phase shift to be applied to an antenna element where the antenna is to produce a beam cluster.
Accordingly, it is the object of the present invention to reduce the peak values of the grating lobes of phased array antennas employing subarrays, thereby improving antenna quality, by providing an aperiodic illumination error function which distributes the signal energy in a random fashion to significantly reduce the peak values of the antenna grating lobes while providing a reliable, relatively uncomplicated, inexpensive antenna.